A calendar mechanism answering generally to the definition which has just been given is known for example from patent documents CH-A-538 136 and CH-A-661 171 (U.S. Pat. No. 4,676,659). In these documents, there is also found a twenty-four hour calendar driving wheel provided with a finger or a long tooth which drives a date disc bearing thirty-one indications. However, the finger does not directly drive the disc, but rather an intermediate wheel set which itself drives such disc. There, to be sure it concerns an ordinary non-perpetual calendar mechanism. At the end of months of thirty days and at the end of the month of February, it is then necessary to effect a manual correction should one not wish to lose the date.
Perpetual calendar mechanisms have already long been known in which an arrangement automatically controls all changes, whether concerning months including 28, 29, 30 or 31 days. Such mechanisms are described in detail in the work entitled "Les montres-calendrier modernes" of B. Humbert--Editions Scriptar S. A. Lausanne 1953 (English language version: "Modern Calendar Watches", Lausanne 1954). Thus, a perpetual calendar watch does not require manual correction of the date display at the end of months counting fewer than thirty-one days. Likewise, every four years upon the occasion of the leap year and in the month of February, the watch will display the figure twenty-nine before indicating the first of March.
Without going into the details, here there will be mentioned a mechanism using a month cam making one rotation in four years. Such cam exhibits notches which are more or less deep: the full portions correspond to the months of thirty-one days; the shallow notches to the months of thirty days; three very deep notches to the months of February in the common years (twenty-eight days) and the notch of intermediate depth to the month of February in the leap year. On such cam there acts the beak of a lever urged by a spring. The depth of penetration of the beak will determine which advance must be impressed on the date indicator at the end of each month.
The mechanism briefly evoked hereinabove uses levers and return springs which lead to a relatively complicated construction which necessitates in turn a relatively high number of parts. On the other hand, it can be mentioned that such mechanisms do not always show reliable operation, particularly if shocks are applied to the timepiece.
To obviate the cited drawbacks, the present invention proposes to place gears only into operation and to exclude all levers or rocking bars, such gears on the one hand being prevented from all untimely rotation, even if shocks are applied to the timepiece and, on the other hand, showing a clearly simplified design and one of reduced height, this being even more so the case since the calendar of the invention is limited to the automatic advance of the date for the months of thirty days only, the resetting of the date having to be manually effected at the end of the month of February. Hence, it concerns an annual and not a perpetual calendar.